The present invention relates to a process for preparing carboxylic acid esters by reacting at least one primary monoalcohol or a mixture of a primary monoalcohol and at least one alcohol different therefrom in the presence of a transition metal carbene complex catalyst K.
Carboxylic acid esters are important chemical compounds which are used for example as solvents, plasticizers, and fragrances and aroma substances. There are various methods for preparing them, the reaction of a carboxylic acid with an alcohol being the most conventional:RaCOOH+HORbRaCOORb+H2O
Since this is an equilibrium reaction which in most cases is acid-catalyzed, the water or the carboxylic acid ester has to be removed from the reaction mixture in order to achieve high yields. Particularly in the case of low-boiling carboxylic acids and low-boiling alcohols, undesired azeotropes are often formed in the process, which hinder separation.
Another way of preparing carboxylic acid esters is the reaction of an acid anhydride with an alcohol:RaCOOCORa+HORbRaCOORb+RaCOOH
In this process one equivalent of the carboxylic acid is formed, which likewise has to be separated off. Here too, azeotropes often form, and moreover two equivalents of the carboxylic acid are required for one equivalent of the carboxylic acid ester.
Another way of preparing carboxylic acid esters is the transition metal complex-catalyzed reaction of alcohols with dehydrogenation, referred to below as direct ester formation. Here, the starting materials used are only alcohols, where at least one of the two reactants must have a primary OH group, i.e. a CH2OH group:RaCH2OH+RbOH→RaCOORb+2H2 
In this type of carboxylic acid ester synthesis, no carboxylic acid need be used, which is advantageous since the lower carboxylic acids in particular have an unpleasant odor. Moreover, no water is formed during the reaction, which simplifies the distillative work-up of the reaction mixture. By virtue of the fact that the reaction conditions can be kept very mild, moreover, only a few by-products are formed.
The conversion of primary alcohols to carboxylic acid esters with catalysis using transition metal complexes is described many times in the literature.
Tetrahedron Lett. 1981, 22, 5327-5330 and J. Org. Chem. 1987, 52, 4319-4327 describe the use of ruthenium complexes with phosphane ligands as catalysts for preparing carboxylic acid esters and lactones from primary alcohols.
J. Organomet. Chem. 1985, 282, C7-C10 describes catalysts for preparing carboxylic acid esters from primary alcohols using a ruthenium complex with tetraphenylcyclopentadienone ligand.
Chem. Rev. 2010, 110, 681-703 describes various transition metal complex catalysts for direct ester formation from primary alcohols.
Organometallics 2011, 30, 2180-2188, Organometallics 2011, 30, 5716-5724 and J. Am. Chem. Soc. 2005, 127, 10840-10841 describe ruthenium pincer complexes for the catalysis of the direct esterification of primary alcohols.
M. Nielsen et al., Angew. Chem. 2012, 124 describes ruthenium and iridium complex catalysts with polydentate organonitrogen and organophosphorus ligands for the synthesis of ethyl acetate from ethanol.
Organometallics 2011, 30, 6044-6048 describes ruthenium complex compounds based on N-heterocyclic carbenes as catalysts in the direct ester formation from primary alcohols. A phosphate is advantageously added to the ruthenium complex.
A disadvantage of the described methods for synthesizing carboxylic acid esters is that the transition metal complex catalysts used have to be prepared by complex syntheses and optionally have to be isolated. Moreover, oxidation-sensitive and expensive phosphane ligands are often used, which necessitates a complex reaction implementation under inert conditions. Furthermore, the procedure is predominantly carried out in relatively large amounts of an added solvent, as a result of which the reaction mixture has to be worked-up in a complex manner and the space-time yield is reduced.
The object of the present invention is to provide a process for the transition-metal-complex-catalyzed direct ester formation from primary monoalcohols or from mixtures of a primary monoalcohol and at least one alcohol different therefrom.